Method of reheating a partially cooled continuously cast slab



June 24, 1969 F. A. ALEXANDER, JR 3,451,664

METH D OF REHEATING A PARTIALLY COOLED CONTINUOUSLY CAST SLAB Sheet L of5 Filed Sept. 21. 1967 FlG.l

INVENTOR. FRANK A. ALEXANDER,JI'.

ATTORNEYS June 24, 1969 v F, A. ALEXANDER, JR 3,451,664

METHOD OF REHEATING A PARTIALLY COOLED CONTINUOUSLY CAST SLAB l I 1' (0LINVENTOR.

FRANK A. ALEXANDER,Jr.

ATTORNEYS Jun 24, 1969 F. A. ALEXANDER'. JR 3,451,654

METHOD OF REHEATING A PARTIALLY COOLED CONTINUOUSLY CAST SLAB Sheet i of3 Filed Sept. 21, 1967 INVENTOR.

FRANK A. ALEXANDER,Jr.

BY M W W ATTORNEXS United States Patent 3,451,664 METHOD OF REHEATING APARTIALLY COOLED CONTINUOUSLY CAST SLAB Frank A. Alexander, Jr., WestBoylston, Mass., asslgnor to Morgan Construction Company, Worcester,Mass.,

a corporation of Massachusetts Filed Sept. 21, 1967, Ser. No. 669,480Int. Cl. F27]: 17/00 US. 'Cl. 263-52 2 Claims ABSTRACT OF THE DISCLOSUREThe invention relates to a process for reheating a slab of metal thathas just been removed from a continuous casting process whereby thereheated slab may then be sent directly from the reheating furnace tothe rolling 1 ing to the rolling temperature. A uniform rollingtemperature throughout the slab is obtained in less than half the timerequired to bring a cold slab up to the same temperature.

Background of the invention This invention will be explained in terms ofthe steel industry, but it will be understood that it is also applicable to other fields, such as copper and aluminum. The invention hasparticular relation to the continuous processing of steel from thecasting stage through the rolling operation. The objective is to takecast steel made by a continuous casting process and then, instead ofsending the casting to storage, to immediately reheat it and feed thereheated casting directly to the rolling mill.

The continuous casting of steel is well understood. The usual procedurefollowing the pouring of the steel into the mold is to cool the surfaceat least enough to produce a rigid shell about the casting (the interiormay still be in molten condition), and then remove the casting after ithas been cut to appropriate length, to a storage position where it willcool for subsequent use. Thereafter, the cooled slab or billet(hereinafter the material will hereferred to as a slab) is taken fromstorage, placed in'a reheating furnace, brought up to the desiredtemperature for the subsequent rolling operation, and then fed to therolls for final processing.

Up to the present, it hasnot been possible to feed a slab produced bythe continuous casting process directly to the rolling mill because thetemperature of the slab following the continuous casting process andinitial essential cooling is not at the uniform temperature required forrolling. In fact, the surface temperature of the slab is so far belowproper rolling temperature, that rolling would 3,451,664 Patented June24, 1969 the still molten interior from melting its way through theouter surface. Therefore, while it is not necessary to cool the entirecontinuously cast slab below the melting point before removal from thecasting machine, it is necessary to have the exterior temperaturesufiiciently lowered so that the still fluid interior cannot escape.

If the continuously cast slab, cut to a suitable length for thesubsequent rolling operation, is to have the heat still present thereinconserved, the slab must be fed immediately to a reheating furnace inwhich the objective would be to allow the still hot interior at perhaps2450? F. to cool to rolling temperature, for example 2300 F., while atthe same time raising the temperature of the exterior parts of the slabfrom a temperature of about 1600 F. to the 2300 F. rolling temperature.

If such reheating should be attempted in 'a conventional reheatingfurnace designed to heat cold billets, it would be found that thecustomary temperatures used therein would heat the exterior of the slabfar too quickly and continue the molten condition of the interior. Theresult would be that the molten interior would melt the exterior shellin spots and escape therefrom.

Accordingly, the present process involves steps which will insuresufliciently slow reheating of the exterior of the slab to rollingtemperature to permit the interior to solidify and the interiortemperature to drop gradually to the rolling temperature. The idealresult is to have all parts of the slab arrive at the desired rollingtemperature at substantially the same time.

Summary (3) Discharging the reheated slab from the furnaceto the rollingmill as soon as uniform rolling temperature has been reached.

Additionally, it will be understood that the output of the reheatingfurnace or furnaces will be geared to the capacity of the rolling millto which the reheated slabs from the continuous casting machine muchfaster than be impossible. No procedure up to the present has been oneevery 70 minutes, it will be necessary to have either a long reheatingfurnace, or several furnaces, it being understood that the output of thecontinuous casting machine will be no greater than the rolling capacityof the rolling train. Alternatively, if the continuous casting machineproduces slabs beyond the capacity of a single rolling train, then twoor more rolling trains may be used. In the practice of the invention,the saving in fuel for reheating is enormous to say nothing of thesaving in inventory, storage space, straightening problems, etc., thatare present when the slabs are stored prior to reheating.

Brief description of the drawings FIG. 1 is a vertical longitudinalsection of a regenerative furnace in which the invention may bepracticed.

FIG. 2 is a vertical section taken on the line 22 of FIG. 1.

FIG. 3 is a plan view taken on the line 3 3 of FIGS. 1 and 2.

Referring to the several figures, the regenerative furnace indicatedgenerally at 2, includes conventional checkers 4 and 6 (see FIG. 2)which alternately (1) receive fresh air to be heated by the checkersbefore mixing with the fuel at 8 to be burned and passed through thefurnace area 10 to heat the slabs 12, and (2) receive the productsofcombustion on the way to the stack thereby to heat the checkers priorto reversal of gas flow. This is conventional.

As shown in FIGS. 1 and 3, there are five sets of checkers, A, B, C, D,and E, side by side, each set having opposed ports for alternatelyproducing a flame serving to heat a slab temporarily at that location.The number of sets of checkers may be more or less depending on thenumber of slabs required to be within the furnace to give the necessaryproduction. The objective achieved by the plurality of checkers is theability to control the quality and temperature of the heat applied toeach slab as it passes through the furnace. Preferably, the slabs willbe moved, step by step, from one set of checkers to the next, butcontinuous movement of the slabs could be used alternatively.

The slabs are brought to the furnace directly from the casting machineon conveyor 14. On reaching entrance 16, the door 18 is raised and theslab 12 is pushed by conventional pushing means (not shown) sidewiseinto the furnace area 20.

As each subsequent slab arrives at door 18, it and all preceding slabsthen in the furnace are pushed the distance of one checker to the left(as viewed in FIG. 1).

As the slabs advance through the furnace, the heat provided at eachsuccessive checker position is controlled to produce a temperaturecondition at the last checker E in which the temperature of the slab issubstantially uniform throughout and correct for immediate rolling.

When the slab 12 reaches exit 22, the door 24 is raised and the slab ispushed out onto delivery rollers 26 which convey it directly to therolling mill.

Further details of the furnace shown are as follows:

FIGS. 1 and 3 show five ports arranged along each side of the furnacelength at the checker positions. These ports in FIG. 1 are numbered 28,30, 32, 34 and 36. Each port connects with a vertical downdraft checker38, a horizontal checker 40, and a vertical updraft checker 42, all inseries. In FIG. 2 is shown an ejector 44 located above each verticalupdraft checker for simplicity of control. Located at the throat 46 ofeach ejector would be a tight shutoff reversing valve (not shown) all ofwhich is conventional. This system would fire alternately from one sidewhile drafting through the opposite side, reversing approximately everythree to four minutes.

The slab 12 on entering the furnace and being placed in alignment withthe ports 28 of the first checkers A will have a skin temperature whichmust be raised while at the same time heat is being soaked out of thecenter to the surface. Experience indicates that one of the best mediumsfor use in heating hot steel is a lean fuel such as blast furnace gas incombination with preheated air having a temperature of 1750 F. to 2000F. This fuel provides a large volume of gas with a relatively low flametemperature in the order of 3000 F. Applying this type of flame to thehot steel slab enables the lower temperature surface to be heatedgradually while soaking the entire piece without weakening thesolidified shell.

Another suitable source of lean gas could be developed by mixingnitrogen with rich fuel. Nitrogen and natural gas in the ratio of sevento one would provide a fuel of about 125 B.t.u.s per cubic foot, withcharacteristics similar to blast furnace gas or lean mixed gas.

The use of a lean fuel producing a low temperature flame results in alight brittle scale on the slab during the reheating process. Since theslabs are preferably resting on refractory supports 27 while beingpushed through the furnace either step by step or continuously, theflame may completely envelop the slab with resulting high uniformity ofslow heat application to the surface while the interior heat is soakingout, thereby avoiding any bleeding of the initially molten interiorthrough the surface skin.

The furnace herein described comprised of five identical checkers, A, B,C, D and E, could reheat 140 tons of cast slabs per hour. The slabs insuch case would be wide, 10" thick and 30 long. If greater productionwere needed, other furnaces could be added, or the single furnacelengthened.

It has been discovered that if the slabs are delivered to the furnacefrom the continuous casting machine with an average temperature of 1700F., the staying time in the furnace would be only approximately 70-75minutes, and a furnace fuel input of approximately 65,000 B.t.u.s perhour would be required at maximum production.

In the system illustrated in the drawing, an ejector 44 has beenindicated for each port. This allows independent control of draft ateach port for varying the quantity of ejection air. When the throatvalve is closed, the airflow is then controlled for combustion purposes.All valves are located on the cold side of the system. This arrangementis ideal since it provides the capability of changing from lean to richgas if necessary.

While the foregoinng description has been directed to the reheating ofslabs, it will be appreciated that reheating of billets delivered to thefurnace in hot condition could also be readily accomplished. The billetswould normally be delivered to the furnace area at a temperaturesomewhat below the temperature of the heretofore referred to slab. Thisaverage temperature would be about l550 F. The billets delivered to thefurnace might be handled in small groups. For example, with afour-strand billet casting machine, a group of four billets indicated at50, 52, 54 and 56 on the conveyor 14 in FIG. 3 would arrivesimultaneously at the furnace. These would be lined up in a pack." Eachpack of billets would be moved as a group through the furnace in muchthe same manner as the slabs. At the discharge end of the furnace, thebillets, if desired, could be separated somewhat by any one of knownmechanical methods and would be arranged for side discharge from thefurnace into the mill.

It is intended to cover all changes and modifications of the examples ofthe invention herein chosen for purposes of the disclosure which do notconstitute departures from the spirit and scope of the invention.

I claim:

1. The method of reheating a slab between a continuous casting processand a rolling process in which, at the commencement of reheating, theexterior temperature of the slab is lower than the interior temperature,the said method functioning to bring the slab to a final uniformtemperature which is intermediate the said original exterior andinterior temperatures, said method comprising the steps of moving a hotand at least externally solidified cutoff slab directly from thecontinuous casting process into a reheating furnace of the regenerativetype, moving the slab through the furnace, and subjecting the slab to asuccession of hot gas sources directed crosswise of the furnace inalternating directions and having a temperature in the order of 3000 F.produced by the use of a lean fuel which in burning generates a largevolume of correspondingly hot gas, whereby the entire slab may beenveloped with said hot gas to raise the surface temperature withoutmelting the surface but still provide time for the internal highertemperature of the slab to work outwardly to produce the saidsubstantially uniform temperature intermediate the original exterior andinterior temperatures in the slab before removal from the furnace.

2. The method of reheating a slab of steel between a continuous castingprocess and a rolling process in which, at the commencement ofreheating, the exterior temperature of the slab is lower than theinterior temperature, to a final uniform temperature intermediate theoriginal exterior and interior temperatures, comprising the steps ofplacing the slab in a regenerative type furnace having a plurality ofside by side sources of flame, advancing the slab from one flame sourceto the next flame source,

subjecting substantially the entire surface of the slab to 10 a flameproduced by the use of a lean fuel having a low flame temperature in theorder of 3000 F., and the rate 6 of advance in relation to the time ofexposure at each flame source being such that When the slab leaves thelast flame source, the temperature of the entire slab will besubstantially uniform and correct for immediate rolling of the slab.

References Cited UNITED STATES PATENTS 2,056,904 10/1936 Morton et a12636 3,385,579 5/1968 Peck et al. 263-6 JOHN J. CAMBY, Primary Examiner.

